The most important commercial process for recovering benzene, toluene and xylene (BTX) aromatic hydrocarbons (HCs) from petroleum streams is liquid-liquid extraction (LLE) using sulfolane or polyalkylene glycol as the extractive solvent. Suitable petroleum streams include reformate, pyrolysis gasoline, coke oven oil, as well as coal tar. Extractive distillation (ED) using N-formyl morpholine as the extractive solvent is used extensively to recover benzene from coal tar and coke oven oil. ED using aqueous sulfolane solvent can recover benzene and toluene from reformate or pyrolysis gasoline after C8+ fractions are removed from the feedstock.
The extractive solvents used in both ED and the LLE aromatics recovery processes are internally circulated indefinitely in a closed loop. Measurable amounts of hydrocarbon (HC) species, that are heavier than the intended feedstock, slip through even well-designed pretreatment units and accompany the feed stream into the extraction process. For a poorly operated or malfunctioned feed pretreatment unit, the level of heavy HCs in the feed stream can be significant. High concentrations of heavy HCs and polymeric materials that are generated by interactions among the heavy HCs, decomposed solvent, solvent additives and species from equipment corrosion lowers solvent performance and can even render the process inoperable.